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authorPiotr Krysiuk <piotras@gmail.com>2021-03-16 08:20:16 +0100
committerDaniel Borkmann <daniel@iogearbox.net>2021-03-17 19:12:43 +0100
commit10d2bb2e6b1d8c4576c56a748f697dbeb8388899 (patch)
tree1c90664adc886dd414a786e8c5b7e06b832b149e /kernel
parentf232326f6966cf2a1d1db7bc917a4ce5f9f55f76 (diff)
bpf: Fix off-by-one for area size in creating mask to left
retrieve_ptr_limit() computes the ptr_limit for registers with stack and map_value type. ptr_limit is the size of the memory area that is still valid / in-bounds from the point of the current position and direction of the operation (add / sub). This size will later be used for masking the operation such that attempting out-of-bounds access in the speculative domain is redirected to remain within the bounds of the current map value. When masking to the right the size is correct, however, when masking to the left, the size is off-by-one which would lead to an incorrect mask and thus incorrect arithmetic operation in the non-speculative domain. Piotr found that if the resulting alu_limit value is zero, then the BPF_MOV32_IMM() from the fixup_bpf_calls() rewrite will end up loading 0xffffffff into AX instead of sign-extending to the full 64 bit range, and as a result, this allows abuse for executing speculatively out-of- bounds loads against 4GB window of address space and thus extracting the contents of kernel memory via side-channel. Fixes: 979d63d50c0c ("bpf: prevent out of bounds speculation on pointer arithmetic") Signed-off-by: Piotr Krysiuk <piotras@gmail.com> Co-developed-by: Daniel Borkmann <daniel@iogearbox.net> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: Alexei Starovoitov <ast@kernel.org>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/bpf/verifier.c4
1 files changed, 2 insertions, 2 deletions
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1c8cbef7cc14..56c63658b702 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5870,13 +5870,13 @@ static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
*/
off = ptr_reg->off + ptr_reg->var_off.value;
if (mask_to_left)
- *ptr_limit = MAX_BPF_STACK + off;
+ *ptr_limit = MAX_BPF_STACK + off + 1;
else
*ptr_limit = -off;
return 0;
case PTR_TO_MAP_VALUE:
if (mask_to_left) {
- *ptr_limit = ptr_reg->umax_value + ptr_reg->off;
+ *ptr_limit = ptr_reg->umax_value + ptr_reg->off + 1;
} else {
off = ptr_reg->smin_value + ptr_reg->off;
*ptr_limit = ptr_reg->map_ptr->value_size - off;